An ion source is a critical component of an ion implanter. The ion source generates an ion beam which passes through the beamline of the ion implanter and is delivered to a semiconductor wafer. The ion source is required to generate a stable, well-defined beam for a variety of different ion species and extraction voltages. In a semiconductor production facility, the ion implanter, including the ion source, is required to operate for extended periods without the need for maintenance or repair.
Ion implanters have conventionally used ion sources with directly heated cathodes, wherein a filament for emitting electrons is mounted in the arc chamber of the ion source and is exposed to the highly corrosive plasma in the arc chamber. Such directly heated cathodes typically include a relatively small diameter wire filament and therefore degrade or fail in the corrosive environment of the arc chamber in a relatively short time. Indirectly heated cathode ion sources have been developed in order to improve ion source lifetimes. An indirectly heated cathode includes a relatively massive cathode which is heated by electron bombardment from a filament and emits electrons thermionically. The filament is isolated from the plasma in the arc chamber and thus has a long lifetime. The relatively massive structure of the cathode ensures operation over an extended period.
Notwithstanding such improvements, ion sources may not provide satisfactory performance under certain operating conditions. State of the art semiconductor devices require extremely shallow junction depths which are obtained with low implant energies. However, ion implanters are typically designed for efficient operation at relatively high implant energies and may not function efficiently at the energies required for shallow junction implantation. At low implant energies, the current delivered to the wafer is much lower than desired and in some cases may be near zero. As a result, extremely long implant times are required to achieve a specified dose, and throughput is adversely effected. A small ion current may be delivered to the wafer because the ion source operates inefficiently at low extraction voltages. In addition, the ion beam expands as it is transported through the beamline of the ion implanter, and ions may strike components of the ion implanter along the beamline rather than the target semiconductor wafer.
Low energy productivity can be significantly improved by running heavier molecules or clusters of atoms. Conventional hot cathode ion sources produce a high output of mono-atoms but tend to break up clusters and molecules, thereby reducing the productivity improvement. An ion source that produces molecules and clusters is disclosed in U.S. Pat. No. 6,452,338 issued Sep. 17, 2002 to Horsky. The disclosed ion source relies on a beam of electrons to ionize molecules and clusters without causing them to break up. The patent describes the range of electron energies as 20-1200 eV, where the highest energies are used to completely break up the molecules and produce multiply charged ions. Extraction currents are limited with this source due to the lack of a plasma within the source.
Semiconductor manufacturers prefer that ion implanters operate over a wide range of implant parameters in order to reduce the need for multiple ion implanters. More specifically, ion implanters should have acceptable performance over a wide range of ion energies, including very low energies to achieve shallow junction depths. Prior art ion sources have had limited operating ranges. Accordingly, there is a need for improved ion sources and methods of generating ions.